1. Field of the Invention
The present invention relates to semiconductor avalanche diodes, and in particular to IMPATT (impact ionisation avalanche transit time) diodes.
2. Discussion of Prior Art
IMPATT diodes employ the impact-ionisation and transit time properties of semiconductor structures to produce negative resistance at microwave frequencies. An IMPATT diode consists of heavily doped n++ and p++ contact regions separated by a depleted region with a doping m profile designed to produce an avalanche region and a drift region. The doping profile is designed to produce an avalanche region with a high electric field, sufficient to generate high multiplication levels by impact ionisation. The doping profile is designed to produce a drift region with an electric field sufficiently high to achieve carrier velocity saturation but sufficiently low to avoid impact ionisation.
A common example of an IMPATT diode has a lo-hi-lo doping profile in which the p++ contact region is followed by a n-type doped region, in which a first layer (adjacent to the p++ contact region) has a low doping concentration (n), a second layer has a high doping concentration (n+) and a third layer has a low doping concentration (n). The first layer is the avalanche region and sustains a high electric field, the second layer is a doping spike to switch the electric field from a high value in the first layer to a lower value in the third layer, which is the drift region. The avalanche region of the diode will break down when the applied reverse bias voltage exceeds a threshold value.
Close to the breakdown voltage a rapid increase in current is caused by avalanche multiplication of holes and electrons in the avalanche region.
If an IMPATT device is mounted in a microwave cavity and a reverse bias voltage close to the breakdown voltage is applied, then the cavity can be tuned to allow the negative resistance of the diode to generate microwave oscillations with the diode voltage swinging above and below the breakdown voltage. When the rf voltage rises above zero (in its positive half cycle), an avalanche is initiated, a small number of holes and electrons arising from the reverse saturation current are greatly multiplied by the avalanche process. IMPATT diodes are normally designed so that the avalanche current peaks as the rf voltage approaches zero (towards the end of its positive half cycle). After passing through the avalanche region the electrons are swept into the low doped drift region and after a transit time delay the electrons are collected at the n++ contact region. Thus, the current resulting from the avalanche transits the drift region for the half period (negative half cycle) when the rf voltage is negative and this yields a negative resistance for rf current.
The IMPATT diode is one of the most powerful solid-state sources of microwave power. Continuous wave (CW) output powers as high as 10 W at a few gigahertz and as high as 1 W at 100 GHz can be obtained from a single IMPATT diode device. However, IMPATT diodes are noisy and sensitive to operating conditions. The noise in an IMPATT diode arises mainly from the statistical nature of the generation rates of electron-hole pairs at and above the breakdown voltage. Noise can be reduced somewhat by operating an IMPATT diode well above the resonant frequency of the diode and keeping the current low. However, these conditions conflict with conditions favouring high power output and efficiency. Examples of IMPATT diodes are discussed in GB2,002,579 and EP757,392.
Partly, because of the high noise associated with IMPATT diodes, three terminal signal generators, such as transistors are preferred at microwave frequencies with subsequent up-conversion and low noise amplification for higher frequencies. However, the high parasitics associated with three terminal structures indicates that two terminal devices, such as IMPATT diodes, would have a natural advantage at microwave and mm-wave frequencies if noise could be reduced.
MITTAT (mixed tunnelling avalanche transit time) IMPATT diodes are also known in which both tunnelling and ionisation effects are strong in the avalanche region, for example in EP262,346 and U.S. Pat. No. 5,466,965. This degrades efficiency as a significant proportion of the generated tunnel current undergoes little or no avalanche multiplication.
The present invention seeks to overcome some of the problems discussed above by providing an IMPATT diode which operates with much reduced noise levels.
According to a first aspect of the present invention there is provided an impact ionisation avalanche transit time (IMPATT) diode device comprising a main avalanche region and a drift region wherein the deuce additionally comprises a narrow bandgap region with a bandgap narrower than the bandgap in the main avalanche region which narrow bandgap region is located adjacent to the main avalanche region in order to generate within the narrow bandgap region a tunnel current which is Injected into the main avalanche region. By incorporating a narrow bandgap region adjacent to the main avalanche region an injection tunnel current pulse can be generated in a predictable manner. This current pulse is injected Into the main avalanche region where a low noise avalanche occurs.
Preferably, the narrow bandgap region is arranged to generate a tunnel current for injection into the main avalanche region at the peak reverse bias voltage of an oscillating voltage applied across the terminals of the diode.
It is preferred that the narrow bandgap region is located at the edge of the main avalanche region.
The doping profile of an IMPATT diode according to the present invention must be designed to achieve an electric field across the narrow bandgap region of sufficient magnitude to provide the desired tunnel current amplitude at the peak reverse bias voltage. For strained semiconductor materials such as Silicon Germanium/Silicon, a plurality of alternating narrow and wide bandgap layers may have to be used to form the narrow bandgap region in order to alleviate strain. However, In unstrained materials such as Galium Arsenide/Aluminium Gallium Arsenide, one narrow bandgap layer may be used to form the narrow bandgap region.
Most of the noise associated with a conventional IMPATT diode occurs due to the statistical nature of the generation of electron-hole pairs during the part of the positive half cycle of the oscillating voltage when the voltage is above the threshold breakdown voltage. The diode structure according to the present invention increases the predictability of electron-hole pairs being generated at voltages above the breakdown voltage and so can enable a low noise narrow pulse of current to be generated close to the time at which the oscillating bias becomes negative.
The IMPATT diode according to the present invention may have a single drift form, for example having a lo-hi-lo doping profile or a Misawa p-i-n doping profile. Alternatively, the diode according to the present invention may be a double drift diode. In a preferred embodiment of the present invention particularly suitable for a single drift diode the narrow bandgap region is located between a heavily doped contact region and the main avalanche region so as to maximise the proportion of the avalanche region which can be used to multiply the electrons generated in the narrow bandgap material. In a preferred embodiment of the present invention particularly suitable for a double drift diode the narrow bandgap region may be located towards the centre of the avalanche region, so that both the n and p components of the tunnel current may undergo avalanche multiplication.
The IMPATT diode according to the present invention may be made of either III-V semiconductor materials, such as Gallium Arsenide/Aluminium Gallium Arsenide, or group IV semiconductor materials, such as Silicon Germanium/Silicon. The thickness of the narrow bandgap region and the composition of the alloys making up the narrow bandgap region are design parameters chosen to achieve the required tunnel current, as will be apparent to the person skilled in the art.
According to a further preferred embodiment of the present invention the length of the drift region (LD) and the length of the avalanche region (LA) are chosen such that the drift region is between 2 and 6 times, and more preferably between 3.5 and 4.5 times, the length of the avalanche region. This ensures that in the fundamental mode of oscillation, the period of oscillation (P) is between 4 and 12 times, and preferably close to 8 times (between 7.5 and 8.5 times), the avalanche region transit time (TA). It has been found that this substantially reduces the noise generated by an IMPATT diode according to the present invention.
The IMPATT diode according to the present invention can be arranged such that at least part of the tunnel current is generated by optical excitation.